1. Field of the Invention
The present invention concerns an imaging device of the type, for example, that associates a scintillator screen with a matrix of photosensitive elements, and which can be be used to convert a radiological image into electrical signals. In particular, the invention concerns means to improve the signal-to-noise ratio during the reading of signals given by the photosensitive elements.
2. Description Of The Prior Art
Conventionally, photosensitive matrices have a network of row conductors and a network of column conductors. At each intersection between a row conductor and a column conductor, there is a photosensitive assembly, hereinafter called a photosensitive dot. The photosensitive dots are thus organized both in rows and in columns. Each photosensitive dot is connected between a row conductor and a column conductor: in fact, to each row conductor there are connected as many photosensitive dots as there are columns of these dots, and to each column conductor there are connected as many photosensitive dots as there are rows of these dots.
The number of photosensitive dots in a given area determines the resolution of the image. There are known ways to make high-capacity matrices of photosensitive elements, for example with 2000.times.2000 photosensitive dots to obtain an image with dimensions of the order of 40 cm.times.40 cm. In this case, each photosensitive dot is located in an elementary zone or surface having maximum dimensions of 200 micrometers.times.200 micrometers.
Each photosensitive dot comprises a photosensitive element such as a photodiode, a phototransistor or a photoconductor; sensitive to visible, light photons. These light photons are converted into electrical charges and these electrical charges get accumulated in an electrical capacitor forming a storage capacitor, formed either by the capacitor of the photosensitive element itself, or by an associated ancillary capacitor. A reading device enables interrogation about the electrical state of the storage capacitor and the conveying of the electrical charge, which forms the signal, towards a signal amplifier.
An example of a photosensitive matrix is given in a French Pat. No. 86 00656, published under No. 2.579.319, which describes, in detail, the working of a photosensitive matrix as well as the appropriate reading method, each photosensitive dot of this matrix being formed by a photodiode in series with a capacitor.
Another French Pat. No. 86 00716, published under No. 2593343, relates to a matrix with a network of photosensitive dots, each formed by a photodiode and a capacitor in series, as mentioned above. This patent describes a method for the fabrication of a photosensitive matrix of this type as well as a method for the reading of this matrix and an application of this matrix to the taking of radiological images. One of the advantages of the type of structure described in this patent is that it enables the making of large matrices, which are therefore advantageously applicable to radiology, in using thin layer deposits of semiconductors, notably amorphous silicon. In this patent, in order to pick up radiological images, the structure presented has a scintillator panel or screen subjected to X-radiation. In response to this X-radiation, the scintillator screen emits a visible light radiation to which the photodiodes are sensitive.
However, one of the main problems in the reading of photosensitive dots, irrespectively of how they are made, and whether the image is a radiological or not, lies in an excessive value of the electrical capacitance displayed by the photosensitive elements. This capacitance extends its influence particularly during the reading of the photosensitive dots, namely during the amplification of the photocharge developed by a photosensitive element, subsequent to its illumination, and this capacitance of the photosensitive elements has the effect of reducing the signal-to-noise ratio.
For, taking for example, a photosensitive point with the structure described in the above-mentioned two patents: the photosensitive dot consists of a photodiode D0 placed in series with a capacitor C0. The photodiode D0 is connected to a given row conductor H0, and the capacitor C0 is connected to a given column conductor V0. All the other photosensitive dots, connected to this same column V0, form an equivalent capacitor Ceq.
The photodiode D0, which is initially reverse biased by appropriate signals applied to the row H0, is illuminated by a light flux emitted by a scintillator. By the application of a suitable electrical signal to the row H0, the potential of a dot A (which is located at the junction of the photodiode D and the capacitor C and at which the photocharge is accumulated) is restored to its initial bias level. The result thereof is the circulation, in the column V0, of an electrical reading charge q0 proportionate to the photocharge, which gives rise to a signal voltage vs at the terminals of the capacitor Ceq of this column.
It can easily be shown that this signal voltage vs is equal to: vs =q0/N.CD; where q0 represents the photocharge developed within the diode D0, N is the number of photosensitive dots arranged along the column and CD is the capacitance of the photodiode.
The signal voltage vs is amplified in an amplifier G which is either directly associated with the column V0 or else associated with several columns by means of a multiplexer device.
As mentioned above, a major characteristic is the signal-to-noise ratio S/N which is related to the capacitance CD of the photosensitive elements by the following relationship: ##EQU1## where vb is the noise voltage at the input of the amplifier.
This relationship shows that the signal-to-noise ratio S/N is optimized when the factor N.CD is minimized, namely that, for a number of photosensitive elements M arranged on a given column, the capacitance of the photosensitive element is the lowest possible.
In the most standard configuration, the capacitance of the photosensitive element, of a photodiode, for example, is related, firstly, to the thickness of the material (intrinsic silicon, thickness limited to a few micrometers) and, secondly, to the active surface or section of the photosensitive diode, which is subjected to luminous photons and is called an "active surface", said surface being demarcated, in practice, by the surfaces of the facing intersecting electrodes, namely by the surfaces of intersection between the row conductors and the column conductors.
Of course, the capacitance of the photosensitive element can be reduced by reducing its active surface, but an arrangement of this type goes against another requirement which is that, to pick up the maximum amount of light coming, for example, from a scintillator screen placed in contact with or near one of the networks of electrodes, or row or column conductors, the photosensitive element must present the maximum possible active surface with the pitches of the networks of the row conductors and column conductors. The pitches of the networks of the row conductors and column conductors divide the surface of the matrix into several elementary surfaces, each having a photosensitive dot, each elementary surface being illuminated by the light coming from a corresponding part of the scintillator, which itself represents an elementary image surface.